1. Field of Invention
The present invention relates to a process for preparing pre-expanded articles of a polyolefin resin having a microcell structure and the pre-expanded articles prepared by the process.
More specifically, the present invention relates to a process for preparing pre-expanded articles in which a hydrocarbon blowing agent such as butane or an inorganic blowing agent is used as a substitute for the freon which has been used until recently as a conventional blowing agent, and furthermore, an impregnating condition of the blowing agent is maintained constantly during the pre-expansion at a certain low temperature range, thereby the pre-expansion being carried out without any anti-adhesion chemical agents or dispersing surfactants.
The present invention also relates to the pre-expanded articles prepared by the process as above mentioned. The pre-expanded articles thus obtained has microcell structure and shows excellent properties in compressive strength and relaxation capability, etc.
2. Background Art
Hitherto, numerous studies relating to the pre-expanded articles of a polyolefin resin and their preparation methods have been carried out in the art. For example, Japanese Patent Publication No. 49040/1985 describes that pre-expansion can be carried out at a temperature in the range of from Tm -25.degree. C. to Tm -5.degree. C. (hereinafter "Tm" means a temperature at which melting of the used resin is terminated) and preparing pre-expanded articles below the temperature of that range is impossible for molding afterwards. And the Publication also describes that the moldability is related to the presence and appearance of the higher melting peak as determined by the differential scanning calorimeter (hereinafter referred to as DSC). This suggests that two melting peaks on DSC curve is important for moldability.
In addition, Japanese Patent Publication No. 107516/1988 describes that the pre-expanded articles can be prepared at a temperature in the range of from Tm +1.degree. C. to Tm +7.degree. C. of the resin used and that when a heat of fusion Q.sub.H for the higher melting peak corresponds to 0.5 to 2.3 cal/g, it is possible to prepare expanded articles without any additional internal pre-pressurization during molding. These pre-expanded articles having not only two distinct melting peaks as determined by DSC but also specific ranges of Q.sub.H for the higher melting peak were claimed to exhibit particular efficiencies in molding.
The aforementioned two publications may be recognized as covering the most broad and basic contents regarding the preparation of pre-expanded polyolefin resin articles. And numerous improvements including them have been made in this art. Most of these inventions, however, use freon gas of halogenated hydrocarbon as a blowing agent, which is well known to a person skilled in the art as the most effective blowing agent for preparing pre-expanded polyolefin resin.
In spite of their effectiveness, the use of the freon based blowing agents is forbidden due to the harmful influence against environment. Therefore, substitute blowing agents such as hydrocarbons or inorganic materials should be used.
The substitute blowing agents, however, have been recognized as having problems such as a lowering of expansion force of the pre-expanded articles (that is, a falling off in expansion ratio) or a lowering of cooling capacity for cell stabilization in contrast to the prior ones. Actually, it has been reported that the cell size of the pre-expanded articles and the cell distribution are substantially irregular when substitute blowing agents instead of freon based blowing agents are used, and the irregularities cause a deterioration in some physical properties.